Process for producing a dried fertilizer from sewage sludge

Abstract

Sewage sludge is subjected to a treatment with an acid in order to form stable ammonium compounds before thermal drying of the sludge which would otherwise volatilize ammonia and diminish the nitrogen content of the sludge.

We claim: 
     
       1. A process, for producing a dried material suitable for use as fertilizer, consisting essentially of (A) obtaining a starting sewage sludge including principally water and a minor amount of solids containing nitrogen as ammonia; and then   (B) mechanically dewatering the material to provide a sludge having about 20% to about 30%, by weight, solids; and then   (C) adding to the dewatered sludge, before its exposure to any temperature as high as about 70° C., an amount of acid selected from the group consisting of nitric acid, phosphoric acid, and mixtures thereof, sufficient to produce a treated material with a pH of about 7 or less, comprising fixed nitrogen in thermally stable compounds which do not volatilize until temperatures over 100° C. are reached selected from the group consisting of ammonium nitrates, ammonium phosphates, and mixtures thereof; and then   (D) thermally drying at temperatures up to about 100° C. to remove water from the treated material, to produce a thermally dried material with a solid concentration of about 70% to about 90%, by weight, where substantially all of the nitrogen of the original sludge remains.   
     
     
       2. The process in accordance with claim 1, wherein ammonia in step (A) is in the form of ammonium hydroxide which has a boiling point of from about 70° C. to about 100° C. 
     
     
       3. The process in accordance with claim 1, wherein the thermally dried sludge produced in step (D) has a nitrogen content containing up to about 5% nitrogen as ammonia, and heating in step (D) is from about 70° C. to about 100° C. 
     
     
       4. A process in accordance with claim 1 further comprising: following thermally removing water, treating the thermally dried sludge by an addition of heavy metal ion fixation agents.   
     
     
       5. A process in accordance with claim 4 wherein: the heavy metal ion fixation agents include a cement and a sodium silicate.   
     
     
       6. A process in accordance with claim 4 further comprising: producing agglomerated soil nutrient pellets from the heated sludge.   
     
     
       7. A process in accordance with claim 6 wherein: the producing of agglomerated soil nutrient pellets is performed simultaneously with the treating by the addition of heavy metal ion fixation agents.